This invention relates to the field of cold cathode charge emitters of the negative electron affinity (NEA) type.
Charge carrier emission of the present invention, e.g. electrons, is to be contrasted with conventional charge carrier emission arrangements such as the thermionic emission commonly employed in vacuum tubes, the Schottky emission employed in certain semiconductor devices, the field emission accomplished with high electric fields stress and generally high voltage potentials, photoelectric emission as employed in vacuum tube photo responsive devices such as the photomultiplier tube, and secondary emission wherein collisions between free and bound electrons result in a generation of additional free electrons.
By way of introducing the present invention a thermionic energy conversion apparatus may be first considered. The thermionic energy converter is an apparatus based upon the Richardson equation; EQU J"=(4.pi.emk.sup.2 /h.sup.3)T.sup.2 e.sup.-.PHI./kT Amps/hu 2 (1)
where h is the Planck constant, m is the electronic mass, T is the electrode temperature, and .PHI. is the electrode work function. The constant at the equation front is the same for all metals, so that EQU J"=12.PHI.T.sup.2 e.sup.-.PHI./kT Amps/cm.sup.2. (1a)
A thermionic converter involves two electrode surfaces placed closed to each other; with one such electrode elevated in temperature above the other this hotter electrode emits more electrons than the cooler electrode. An electrical load connected to the two electrodes allows such a thermionic converter to be used as an electrical power source.
In such thermionic converters, as are shown in FIG. 2A and 2B of the drawings bare metals disposed in a vacuum can produce only small amounts of electric power because the work function of virtually all metals is too high to allow large amounts of emission and because as electrons are emitted from an emitter, a large negative potential forms in the inter-electrode space so that additional emissions are prohibited. This latter effect is especially strong unless the emitter to collector gap is small and on the order of ten microns or less. Both of these limitations are compensated by the introduction of cesium into the inter electrode gap however. Such cesium adsorbs onto the surface of the electrodes and creates a much lower work function of about 1.4 to 1.6 electron volts for collectors operating in the 800 to 1000 degree Kelvin temperature range. The low work function of bulk cesium i.e. about 1.8 electron volts and the formation of a dipole layer are due to the donation of electrons from the cesium to the metal substrate. Both of these factors contribute to a lower effective work function in a thermiomic convertor.
If however, a thermiomic convertor of this type is fabricated with graphite electrodes it has been found possible to observe unexpectedly high current density in the thermionic graphite electrodes during reverse bias operation with an applied voltage of several volts. It is believed that the n-p junction flooding phenomenon as described below (and as leads to the present invention) is a definitive explanation for these high current densities.
The existence of anomalously high current densities in plasmas attending the electrodes of a thermiomic convertor suggests of course the possibility of using the attending electron generation mechanism as a cold cathode source of electrons or as a cold cathode free electron generation source. Such a cold cathode free electron source is distinguished from the above recited conventional sources of electrons, that is, from thermiomic emission, Schottky emission, field emission, photoelectric emission and secondary emission.
The patent art indicates considerable activity relating to the generation of electrons. The patents resulting from this activity include U.S. Pat. No. 5,141,460 issued to J. E. Jaskie et all, U.S. Pat. No. 5,129,850 issued to R.C. Kane et all and U.S. Pat. No. 4,307,507 issued to H. F. Gray et all. Since each of these patents is concerned with field emission apparatus and the field emission phenomenon in general, the present negative electron affinity (NEA) related invention is readily distinguished from the disclosure of these patents.
Additional patent art is of background interest with respect to the present invention; this art includes U.S. Pat. No. 5,074,456 issued to R. L. Degner et all and concerned with a composite electrode for use in a plasma process. The Degner et all patent discloses that electrodes for plasma reactors have been formed from a material such as graphite and that such material may be purified to semiconductor purity. The Degner et all disclosure however does not teach the injection of n type charge carriers into the graphite material as is accomplished in the present invention. In addition, the U.S. Pat. No. 4,774,991 of J. A. Holden is concerned with the formation of a rotary grinding wheel dresser in which diamond particles are adhered to an internal graphite ceramic or metal surface. The Holden disclosure is however distinct from the present invention in that a cathode structure is not formed and the material surrounding the diamond is not an n-type charge carrier.
The U.S. Pat. No. 4,277,293 of R. S. Nelson et all is also of interest with respect to the present invention. This Nelson et all patent is concerned with a method for growing synthetic diamond crystal having increased electrical conductivity with respect to normal diamond crystal and involves the bonbardment of diamond crystals with a flux of high energy carbon ions. The Nelson patent diamond, however has a damaged crystal structure which includes so many defects as to be unsuited to the high quality diamond crystal requirements needed in applicant's electron emitter invention. The diamond crystal of applicant's invention remains of high latice integrity notwithstanding the carrier flooding accomplished from an adjacent p-n junction; this high quality diamond in readily distinguished over the bombarded crystal diamond disclosed in the Nelson patent.
In addition to these patent documents, two technical periodical publications by one M. W. Geis et all have suggested that diamond may be used as a cold cathode material in view of the available negative electron affinity characteristics of diamond. These Geis et all publications include the article "Diamond Cold Cathodes" presented at the Second International Symposium on Diamond Materials conducted by the Electro Chemical Society of 10 South Main St. Pennington, N.J. 08534-2896 and the related technical article "Diamond Cold Cathode" published in the Institute of Electrical and Electronic Engineers Electron Device Letters Volume 12, No. 8 and dated Aug. 1991.
The Geis et all electron emitter requires the fabrication of n-type diamond and therefore requires special fabrication techniques such as ion implantation of carbon ions. Such implantation is known to normally introduce undesirable side effects such as increased latice defect density and reduced electron mean free path length. The p-type diamond of the present invention is readily obtained from a diamond film growth process, but requires an injection of charge carriers from an adjacent n-type material as is described below and as is not employed by Geis et all.
Since therefore the Geis et all published articles disclose the use of n-type diamond as an emitting material and the present invention employs p-type diamond in this emitter capacity, the present invention is readily distinguished over the disclosures of Geis et al.
The following publications are also of general background interest with respect to the present invention.
1. The paper "Deep Level Transients Spectroscopy Study of Thin Film Diamond" from the topic Diamond, Silicon Carbide and Related Wide Bandgap Semiconductors, Materials Research Society Symposium Proceedings, vol. 162, Materials Research Society, 1990, pages 309-314.
2. The paper "Experimental Studies on Thermionic Devices with Cesium-Barrium Fillings" published at the Sukhumi Nuclear Power Engineering in Space Conference, Oct. 28-31 1992.
3. "Cold Field Emission from CVD Diamond Films Observed in emission Electron Microscopy," published in Electronic Letters 1 Aug. 1991 vol. 27 no. 16 pages 1459-1460.